I do. Almost all the time. If someone breaks into my truck to steal sh|t I'm not going to feel bad if they get my gun and do something bad with it. Any more than I'll feel bad if they break in my house and do the same. Criminals are criminals... I don't feel I'm offering them any kind of "opportunity" that they wouldn't have if they wanted to. Although I do have a lock box I occasionally use for it, but not regularly, no. I doubt all these members who leave a rifle in their trucks or trunks are going to pull them out and carry them inside every day at work or home, either. My truck is always locked with an alarm and visual indicator (flashing light). I don't feel responsible for criminal acts committed by people who are determined to victimize others. Just me, I guess.
Your choice, I understand your point, but completely disagree with you. I think it's very irresponsible as a gun owner, I hear too many stories of "my gun was stolen out of my car/truck" stories. Alarms deter, but don't stop the break in, they will smash/grab and run for the most minor stuff. Your stuff, your choice, just hate seeing more guns getting into the hands of the bad guys, certainly doesn't help build a strong case as responsible gun owners.